Human hepatocyte growth factor (hereinafter refer to as “HGF”) has been purified as a liver regeneration factor. A gene encoding this factor has also been cloned and the sequence has been determined. Initially, HGF had been considered to function only for hepatocyte growth. However, subsequent studies have revealed that HGF does not only function for growth and regeneration of a hepatocyte, but also has strong effects of protecting from damage and of regenerating an organ on lung, kidney, blood vessel and heart tissues. Moreover, HGF also has very varied functions, for example it shows a strong antitumor activity against certain types of cancer.
From a study using various types of cultured cells, it has been found that HGF functions as a growth promoting factor, a mobility promoting factor, a morphogenesis promoting factor and a tumor suppressing factor. Moreover, expression of HGF enhances in organs, such as the lung and the kidney, responses to hepatopathy, and regeneration of the liver is promoted by a mechanism via the blood. It has been confirmed that, in other organs such as the kidney or the lung also, regeneration of such organs is promoted by the same mechanism. All of these HGF functions are biological activities essential for the construction and maintenance of tissues and organs, and so it is expected that HGF would be clinically significant when applied as specific medicines for intractable organ diseases for which basic treatment methods have not yet been established. Furthermore, a gene therapy for chronic arteriosclerosis obliterans of diabetes patients, which uses an HGF gene, is being attempted.
It has been reported that HGF has many variants generated by alternative splicing. It has been shown that, of these, a variant HGF which lacks 15 base pairs in the first kringle domain corresponding to a receptor binding site, that is, a variant HGF which lacks 5 amino acids, has a two or three times higher growth promoting activity on epithelium cells, when compared with ordinary HGF, and that this variant HGF has a different physiological action. It is hoped that this 15 base pairs-deleted HGF has a higher treatment effect on diseases mainly such as damaged epithelial tissues.
A mechanism for regenerating liver tissues by HGF will be described in detail. The expression of HGF mRNA is induced promptly in an interstitial cell, such as the Kupffer cell or the sinusoid endothelial cell in the liver, in response to various types of hepatopathies. HGF produced and secreted from interstitial cells acts on an epithelial cell such as a hepatocyte or biliary cell and promotes regeneration of the liver. Experiments have been carried out where recombinant HGF was administered to a disease model animal, and it has been reported that the recombinant HGF regenerated many types of organopathy. It has been reported that the tissues of many impaired organs such as the liver (e.g. hepatocirrhosis, hepatitis, fatty liver disease, etc.), the kidney (acute and chronic renal failure), the lung, the heart and the stomach, have been regenerated.
The full length of a human HGF gene (hereinafter, referred to as “hHGF”) spans about 70 kb. The full length of mRNA, which is a transcribed product of the hHGF gene, is about 6 kb, and the length of a region encoding a protein in the mRNA is about 2.2 kb. The hHGF is initially synthesized as a single prepro-HGF consisting of 728 amino acids, and after 31 amino acids existing at the N-terminus are cleaved, a portion between the 494th Arg and the 495th Val is cleaved with protease, so that it becomes a mature molecule in which the a and β chains link with a single difulfide bond.
Thus far, HGF genes of human, mouse, rat and so on have been cloned, and the nucleotide sequences thereof have been determined.
With the recent trend of the aging of household pets, there occurs a problem regarding increases of various atrophic or regressive canine diseases, which are associated with aging. The development of pharmaceuticals directed towards such diseases, in which the regeneration of tissues are required, is considered to be important. Generally, these diseases often become chronic. Long-term administration of medicine is required to treat chronic diseases. However, if recombinant HGF proteins are administered to different species, the problem of antigenicity occurs and there may be a risk that long-term administration becomes impossible. Therefore, a recombinant canine HGF, which does not have the problem of antigenicity and can be administered for a long time, is required as a therapeutic agent for these chronic canine diseases.